The present invention relates to a vehicle brake system having an anti-lock control function for preventing lockup of the vehicle wheels while the brakes are being applied to the wheels.
Among such known vehicle brake systems, there are ones comprising a friction mechanism for applying frictional braking force to vehicle wheels and a motor for applying regenerative braking force to vehicle wheels. The motor applies regenerative braking force to a plurality of wheels, typically to drive wheels, all at once. Thus, as soon as the anti-lock control, which is performed on individual wheels, starts, the regenerative braking function is disabled, and the anti-lock control is performed based solely on the frictional braking force.
In order to prevent a sudden and sharp fall of the vehicle deceleration due to the regenerative braking being disabled at the start of anti-lock control, JP patent publication 2002-356151A proposes to reduce the regenerative braking force before the start of the anti-lock control and simultaneously increase the frictional braking force corresponding to the reduction of the regenerative braking force.
In this arrangement, there is the possibility that the regenerative braking force may decrease to zero before the start of the anti-lock control and as a result, only the frictional braking force is applied to the wheels. If the anti-lock control starts in this state, the frictional braking force is reduced to eliminate the locking tendency of a wheel. Compared to electrically generated regenerative braking force, mechanically generated frictional braking force cannot be sufficient quickly increased or reduced in response to a control signal. Thus, it takes a rather long time to reduce frictional braking force until the locking tendency of the wheel is eliminated. This could destabilize the behavior of the vehicle. Also, if the regenerative braking force drops to zero before the start of the anti-lock control, it becomes impossible to recover energy by regenerative braking, thus lowering energy efficiency of the vehicle.